C,C-diacetylenic phosphaalkenes in palladium-catalyzed cross-coupling reactions

Isomerism and dynamic behavior of bridging phosphaalkynes bound to a dicopper complex

A dicopper complex featuring a symmetrically bridging nitrile ligand and supported by a binucleating naphthyridine-based ligand, [Cu₂(μ-η ¹ :η ¹ -MeCN)DPFN](NTf₂)₂, was treated with phosphaalkynes (RC[triple bond, length as m-dash]P, isoelectronic analogues of nitriles) to yield dicopper complexes that exhibit phosphaalkynes in rare μ-η ²:η ² binding coordination modes. X-ray crystallography revealed that these unusual "tilted" structures exist in two isomeric forms (R "up" vs. R "sideways"), depending on the steric profile of the phosphaalkyne's alkyl group (R = Me, Ad, or ^t Bu). Only one isomer is observed in both solution and the solid state for R = Me (sideways) and ^t Bu (up). With intermediate steric bulk (R = Ad), the energy difference between the two geometries is small enough that both are observed in solution, and NMR spectroscopy and computations indicate that the solid-state structure corresponds to the minor isomer observed in solution. Meanwhile, treatment of [Cu₂(μ-η ¹:η ¹-MeCN)DPFN](NTf₂)₂ with 2-butyne affords [Cu₂(μ-η ²:η ²-(MeC[triple bond, length as m-dash]CMe))DPFN](NTf₂)₂: its similar ligand geometry demonstrates that the tilted μ-η ²:η ² binding mode is not limited to phosphaalkynes but reflects a more general trend, which can be rationalized via an NBO analysis showing maximization of π-backbonding.

Direct, Sequential, and Stereoselective Alkynylation of C,C-Dibromophosphaalkenes

The first direct alkynylation of C,C-dibromophosphaalkenes by a reaction with sulfonylacetylenes is reported. Alkynylation proceeds selectively in the trans position relative to the P substituent to afford bromoethynylphosphaalkenes. Owing to the absence of transition metals in the procedure, the previously observed conversion of dibromophosphaalkenes into phosphaalkynes through the phosphorus analog of the Fritsch-Buttenberg-Wiechell rearrangement is thus suppressed. The bromoethynylphosphaalkenes can subsequently be converted to C,C-diacetylenic, cross-conjugated phosphaalkenes by following a Sonogashira coupling protocol in good overall yields. By using the newly described method, full control over the stereochemistry at the P=C double bond is achieved. The substrate scope of this reaction is demonstrated for different dibromophosphaalkenes as well as different sulfonylacetylenes.

Phosphorus centers of different hybridization in phosphaalkene-substituted phospholes

Phosphole-substituted phosphaalkenes (PPAs) of the general formula Mes*P=C(CH(3))-(C(4)H(2)P(Ph))-R 5 a-c (Mes*=2,4,6-tBu(3)Ph; R=2-pyridyl (a), 2-thienyl (b), phenyl (c)) have been prepared from octa-1,7-diyne-substituted phosphaalkenes by utilizing the Fagan-Nugent route. The presence of two differently hybridized phosphorus centers (σ(2) ,λ(3) and σ(3) ,λ(3)) in 5 offers the possibility to selectively tune the HOMO-LUMO gap of the compounds by utilizing the different reactivity of the two phosphorus heteroatoms. Oxidation of 5 a-c by sulfur proceeds exclusively at the σ(3) ,λ(3) -phosphorus atom, thus giving rise to the corresponding thioxophospholes 6 a-c. Similarly, 5 a is selectively coordinated by AuCl at the σ(3),λ(3) -phosphorus atom. Subsequent second AuCl coordination at the σ(2),λ(3) -phosphorus heteroatom results in a dimetallic species that is characterized by a gold-gold interaction that provokes a change in π conjugation. Spectroscopic, electrochemical, and theoretical investigations show that the phosphaalkene and the phosphole both have a sizable impact on the electronic properties of the compounds. The presence of the phosphaalkene unit induces a decrease of the HOMO-LUMO gap relative to reference phosphole-containing π systems that lack a P=C substituent.

Alternative Synthesis and Structures of C-monoacetylenic Phosphaalkenes

An alternative synthesis of C-monoacetylenic phosphaalkenes trans-Mes*P=C(Me)(C≡CR) (Mes* = 2, 4, 6-^tBu₃Ph, R = Ph, SiMe₃) from C-bromophosphaalkenes cis-Mes*P=C(Me)Br using standard Sonogashira coupling conditions is described. Crystallographic studies confirm cis-trans isomerization of the P=C double bond during Pd-catalyzed cross coupling, leading exclusively to trans-acetylenic phosphaalkenes. Crystallographic studies of all synthesized compounds reveal the extend of π-conjugation over the acetylene and P=C π-systems.